Plateaus

Plateaus – Plateaus are “flat or nearly flat elevations of considerable areal extent, dropping off abruptly on one or more sides” (IHO, 2008). Areas of flat ocean floor that are raised above the level of surrounding seafloor, delimited by a steep slope, are known as submarine plateaus. The plate tectonic theory explains how the rifting apart of ancient super-continents, Gondwanaland and Laurasia, over the past 100 million years has resulted in the present day configuration of ocean basins and continents. The disintegration of Gondwanaland not only created the six large continents, but also a number of smaller continental fragments that are scattered around the globe. Some of these so-called micro continents rise above the ocean surface forming large islands like New Zealand; others remain entirely submerged, forming the largest, broad undersea plateaus.

A total of 184 plateaus were mapped in this study, covering an area of 18,486,600 km2, or 5.11% of the oceans. The largest plateau is located in the South Pacific Ocean, extending from New Zealand to northeast Australia, including Challenger Plateau and Lord Howe Rise (Harris, 2011) and covers a total area of 1,505,370 km2. Other plateaus of notable size are the Campbell Plateau (1,229,370 km2) and the Kerguelen Plateau (1,226,230 km2). Plateaus are generally important features in the South Pacific and Indian Oceans, where they cover areas of over 7% and 8% of those ocean regions, respectively. There were no plateaus mapped in the Mediterranean and Black Seas in this study.

Other plateaus are produced from large volcanic eruptions or from tectonic uplift of ocean crust. Plateaus are normally draped by sediments, but since the tops of plateaus are cut-off from receiving any sediment from the continents because of their raised profile, the only food source to the benthos is derived from the rain of pelagic organic matter from the overlying water column.

An example of a large oceanic plateau is the Lord Howe Rise located in the Tasman Sea between Australia and New Zealand (Harris, 2011). The plateau is a fragment of continental crust that separated from Australia around 40 million years ago. It has subsided to a depth of around 1,500 m and because the overlying ocean is oligotrophic, pelagic sediments are accumulating on its flat top at a sluggish rate of only a few centimetres per thousand years. It is characterised mostly (95%) by sediment infauna with isolated volcanic cones providing hard substrate for filter-feeding epifauna.

bathymetry and benthic samples collected from Hatton Bank, between 600 and 2,000 m water depth. Benthic communities are sparse where the seabed comprises mobile sediments (contourite drift deposits) in deeper water, whereas cold-water corals are common on rocky outcrops closer to the top of the bank.

Nodder et al. (2012) describe the benthic communities of the Chatham Rise, a 160,000 km2 submarine continental ridge that extends eastward from New Zealand into the Southwest Pacific Ocean in water depths of  50 to  2,000 m. Isolated groups of volcanic seamounts are present on the flanks of the rise. Most of the Chatham Rise consists of sediment substrates populated by mobile fauna, with conspicuous examples including scampi (Metanephrops challengeri ), squat lobsters (Munida gracilis ), several crab species, quill worms (Hyalinoecia tubicola ), urchins (Parametia peloria ), and other spatangids and asteroids.